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Summary Investigating the mechanisms of various attacks in vehicular ad hoc networks (VANETs) provides fundamental basis to develop safeguard techniques for network security issues. As they develops, attacking strategies are also needed to be established for people to learn how to defend. Previous works failed to consider multiple victims recovery and minimum road side units (RSUs) deployment issues. In this paper, we approach the attacking strategy from an adversarial point of view and develop a trajectory tracking and recovering attack based on matrix completion (TTRA). By randomly sampling locations of vehicles, TTRA is capable of recovering locations of any user with tolerable deviation. After that, the problem of how to minimize the number of RSUs while tracking all vehicles in the whole region is also considered by converting this problem into set covering problem. A heuristic algorithm based on hierarchical clustering is proposed accordingly. To the best of our knowledge, this is the first attempt of recovering mobile users' trajectories by applying these novel techniques. Finally, simulation analysis is used to show the performance of the proposed scheme. Simulation results demonstrate the merits of the scheme in terms of tracking results, location error, and influences of sampling ratio, among others.

Visible Light Communication (VLC) is a promising technology for future wireless communications. By modulating the visible lightthat has about 10,000x larger frequency band than that of radios-to transmit data, VLC has the potential to provide ultra-high-speed wireless connectivities. However, it also has limitations such as i) surrounding objects can easily block VLC links, and ii) intense ambient light can saturate the photodiodes of VLC receivers.In this work, from a different angle compared with state-ofthe-art solutions, we utilize the side channel of VLC-a Radio Frequency (RF) channel created unintentionally during the transmission process of VLC-to break the above-mentioned VLC limitations.The key enabler is that the side RF channel also contains the data information transmitted in the VLC link. When the VLC link is blocked or saturated, we can utilize the side channel, capable of penetrating through blockages and not affected by ambient light, to assist VLC transmissions. Thus a user service relying on VLC transmissions will not be interrupted. Besides the simple Single-Input Single-Output (SISO) case, we consider challenging scenarios where multiple VLC chains are synchronized to form Multiple-Input Multiple/Single-Output (MIMO/MISO) transmission strategies. To make our system practical, we address several challenges spanning from hardware to software. Compared to state-of-the-art design, we reduce the size of the receiving coil by nearly 90%. Experimental evaluations show that our system can decode overlapped RF signals created by a 3×3 MIMO VLC network five meters away, with various blockages in between. Our system also works under intense ambient light conditions (>100,000 lux).

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